Probably a bit late now but as I said, your contract was with the dealer and not the amp manufacturer. They should have been able to offer you ANY amp that they stock and it should have been equivalent value with your input taken into consideration.
They'll send that streamliner back and get a full credit for the trade price they paid for it. I don't know the margins in the music trade but I bet trade price is about 50/60% of full retail. And your Genz will be used as a parts supply for other warranty issues they receive so it has a value to them.
If it was bought in the Genz clear out it probably was about £300 trade, if it was before that it will be a load more - maybe £450 I am guessing? The Rumble will have cost them significantly less than either - maybe £200.
Either way, if your happy with your new amp then all is good ☺